1. Field
This invention relates to devices and methods for replacing damaged spark plug bores in internal combustion engines, and in particular, to permanently installed thread inserts and methods for installing the thread inserts.
2. State of the Art
The threads of a spark plug bore in a cylinder head can be damaged if the spark plug is over torqued or misaligned during installation or is not tightened sufficiently during installation allowing the spark plug to loosen during use. The threads are either stripped entirely or become cross threaded. This problem is compounded by the modern trend to construct cylinder heads from aluminum, a relatively soft metal, and from the reduction of the number of threads in spark plug bores. This is a particular problem with aluminum heads used in Ford engines, particularly the modular engines such as the V-10 5.4 and 4.6 models. The spark plug bores in these engines have only about four full threads.
Damaged threads are particularly troublesome if the spark plug bore is designed for a tapered seat spark plug as in the indicated Ford engines, in lieu of the traditional gasket seat spark plug. Tapered seat spark plugs require that the spark plug bore have a special tapered seat in and as part of the bore whereby an air tight seal is formed between the spark plug and the cylinder head. When the threads are damaged, the tapered seat spark plug may not sit properly in the bore which prevents the air tight seal from forming. A faulty airtight seal can cause a loss of combustion pressure in the engine. A faulty airtight seal can also allow carbon tracking, the build up of conductive carbon residue on the wall of the spark plug bore orifice caused by the escaping gases. Carbon tracking can cause the spark plug or spark plug wire to short out leading to either no spark or a weakened spark in the cylinder. Both the loss of combustion pressure and carbon tracking results in unsatisfactory engine performance.
One current repair method, and the method currently recommended by Ford for the indicated engines, consists of replacing the cylinder head having the damaged spark plug bore with a new cylinder head. This is extremely costly to a consumer because the cylinder heads are expensive to purchase and difficult and expensive to replace.
A cheaper repair method currently in use for some engines where the threads of the spark plug bore are easily accessible and which is well known in the art consists of installing commercially available thread repair inserts, such as a helical insert. These types of inserts require that the damaged bore be drilled out to a larger diameter and re-threaded with a commercially available tap. The insert is then threaded into the bore using the newly tapped threads. The threaded inner bore of the insert then serves as the new threads for the spark plug. However, the prior art thread inserts do not replace the tapered seat. This is important because drilling and tapping a tapered seat spark plug bore destroys the tapered seat in the bore. The original tapered seat plug cannot be used and must now be replaced with a gasket seat spark plug.
This leads to two complications. First, the non-original type spark plug can lead to inferior engine performance. This is due to the fact that the non-original type plug may not have the same characteristics as the original plug. For example, the non-original type plug may have a shorter or longer reach than the original plug. A longer reach spark plug could cause spark knock along with residue filled threads which can cause difficulties in spark plug removal. This also can possibly damage the piston as well. A shorter reach spark plug causes the thread insert""s threads to fill with residue allowing the spark plug to easily foul and misfire. The other complication is that the consumer could become confused from having a mismatched set of spark plugs leading to the installation of a plug that could cause engine damage.
The problems are compounded with the Ford engines mentioned because the spark plug bores are extremely deep so the spark plug is installed in the lower portion of the bore and is completely within the bore and only accessible from the top opening of the bore. This means that the damaged threads will be at the bottom end of the bore and very difficult to get to. This makes successful installation of a prior art thread insert very difficult. This leads to Ford""s recommendation of replacement of the cylinder head to repair damaged threads in the spark plug bore.
Accordingly, there has existed a need for a thread insert that can replace both the threads and tapered seat in a damaged spark plug bore, which is relatively simple and easy to install, and that will allow the use of the original, factory specified spark plug after the repair is complete. This is particularly needed in the aforementioned Ford engines where the threads are located at the bottom of a very deep spark plug bore so are very difficult to access.
According to the invention, a thread insert is provided that replaces both the threads and tapered seat of a damaged tapered seat spark plug bore. Unlike prior art thread replacement devices and methods, the thread insert and method of the present invention allows the repair of a damaged tapered seat spark plug bore in deep spark plug bores without the needless expense and headache of removing and replacing the cylinder head or the use of an insert, such as a xe2x80x9chelicoilxe2x80x9d helical insert, that does not allow the use of the original type of spark plugs. The thread insert allows the spark plug to be positioned in the spark plug bore to extend to substantially the same depth in the cylinder as it would have in the original bore thus eliminating the debilitating effects of over or under reach spark plugs. Further, a damaged spark plug bore in an aluminum cylinder head repaired with the thread insert is superior to the stock head configuration because the thread insert is much less susceptible to damage from cross threading or stripping since it is made from a harder metal than aluminum, such as steel or a steel alloy.
The thread insert of the invention has an exterior threaded shaft extending from a flanged head to a distal end. A non-threaded and recessed neck is preferably interposed between the exterior threaded shaft and the flanged head. The recessed, non-threaded neck area serves as a collector for excess bonding agent, such as J-B Qwik Weld, which can be applied to the exterior threads of the insert during installation to bond the insert to the cylinder head. This securely holds the insert in place to allow a spark plug to be removed from the insert while the insert remains in the cylinder head. It also helps form an airtight seal between the thread insert and cylinder head. The flanged insert head positions the insert in the cylinder head and prevents the thread insert from being threaded too far into the cylinder, which could cause damage to the piston or unsatisfactory spark plug performance.
The thread insert has an inner bore configured to reproduce the undamaged tapered seat spark plug bore. In particular, the inner bore has a threaded portion toward the distal end to receive the threaded end of the spark plug. A non-threaded portion in the inner bore is disposed above the threaded portion of the inner bore and the top of the thread insert and includes a taper formed in the insert to mate and seal with the tapered seat of the spark plug. The non-threaded portion and taper is contoured to the same specifications as the original undamaged spark plug bore. Thus, the non-threaded portion and taper forms a new tapered seat whereby an airtight seal is created with the spark plug such that it is able to withstand the high pressures of an internal combustion engine. Further, the dimensions of the thread insert are such that when installed into the cylinder head, the spark plug inserted into the thread insert is at substantially the same depth in the cylinder as it would have been in the undamaged bore.
The thread insert may be formed of various materials, preferably a material harder than aluminum, such as steel or a steel alloy, which will better resist thread damage than aluminum. The thread insert may be adapted to conform to different engine designs. In adapting the thread insert, the length of the exterior threaded shaft must be such that it does not extend into the cylinder when inserted. Also, the shape of the inner bore is easily determined since it is governed by the specifications of the tapered seat spark plug recommended by the engine manufacturer.
The method of installing the thread insert requires that the damaged spark plug bore be drilled and tapped as is well known in the art. The only special consideration is the use of guide pilots to ensure correct alignment in the cylinder head and that the piston head is not damaged. The guide pilots are specifically adapted for each type of engine. The exterior of the guide pilot is cylindrical in nature with the lower portion having a slightly smaller diameter than that of the upper portion. The lower portion of the guide pilot has a smaller diameter in order for it to snugly slide into the upper portion of the spark plug bore. The upper portion of the guide pilot, also being of cylindrical shape but having a larger diameter than the lower portion, has one side flattened. The flattened side abuts against the cylinder head such that the guide pilot is properly aligned over the spark plug bore. The flattened side also prevents the guide pilot from spinning during drilling or tapping.
The guide pilot has a smooth inner bore having a diameter just larger than that of a drill bit or tap of a particular size, allowing the bit or tap to slide through but preventing them from becoming misaligned. The drill bits and taps can be further adapted for use with the guide pilots by installing a stop pin or other similar device onto their shafts. The stop pin prevents the bit or tap from damaging the piston by stopping the bit or tap before entering the cylinder too deeply.
The thread insert can be included in a kit of interrelated parts, which includes: guide pilots, drill bits and taps adapted to be used with the guide pilots, and a bonding agent.